Hey, a visitor! Don't get those too often. I don't play OU that much, but here are my observations.
Well, TTar is a weather starter in a weather generation, so that says something. However, unlike Politoed, Ninetales, and Abomasnow, the niche isn't completely exclusive since Hippowdon can also summon sand. Thankfully, Hippo is different enough (e.g. they sponge different sides of the spectrum) where TTar still is a great Pokemon. However, there are a few significant downfalls. Namely, his typing is rather disappointing. While it does make him arguably the best Pursuit user in the tier, his other STABs of choice are rather lacking. Compared to things like Terrakion and Salamence, who boast 120 BP STABs, Tyranitar only has the weak Crunch and inaccurate Stone Edge. Defensively, its even worse, as despite a couple nifty resistances, his weaknesses to Fighting (with tons of Fighting-types available), Ground (Duggy is used quite a bit to trap weather starters, and both Landorus forms are great), Water (so much rain), and Bug (U-Turn hurts) make it hard sometimes to bring him in and keep him healthy.
Overall though, despite bad defensive typing negating some of his bulk, his unpredictability (he has quite the movepool iirc), effective use of Pursuit, and most importantly ability to set up weather make him a great Pokemon to use. In viability ranking terms, I would put him in mid-to-high A, although I don't usually like much in S anyway.